There is groundwater everywhere on earth, but our ability to pump it and use it varies from place to place, depending on rainfall and geology- the rock and sand layers within whose pore spaces the groundwater sits. Generally, groundwater is renewed only during a part of each year through precipitation, but can be withdrawn year-round serving as a natural reservoir. Provided that the withdrawal rate does not exceed the rate at which groundwater is replenished, and that the source is protected from pollution, groundwater can be abstracted indefinitely.
The groundwater cycle in humid and arid regions differ fundamentally from each other. In humid climates like ours, with high rainfall, large volumes of water seep into the groundwater, which contributes actively to the water cycle feeding streams, springs and wetlands during periods when the rainfall is lower. In semi-arid and arid climates, there is by contrast practically extremely limited exchange between the surface water and groundwater because the small volume of seepage from the occasional rainfall only rarely penetrates the thick and dry (unsaturated) soils. The groundwater is much deeper and isolated from surface contact. In these areas groundwater resources are only minimally recharged. Our understanding of the complete water cycle is still only rudimentary.
We do know that groundwater availability varies by location. Precipitation and soil type determines how much the shallower groundwater is recharged annually, and the volume of water that can be stored is controlled by the reservoir characteristics of the subsurface rocks. Groundwater may be present today even in places with very dry climates because of the nature of the local geology and the historic climate cycles that have occurred through time. Giant groundwater deposits of limited recharge are thought to exist on nearly all continents, but the amount of groundwater that can be pumped out is unknown. Scientists are just beginning to study deeper sources of groundwater.
Groundwater is usually cleaner than surface water. Groundwater is typically protected against contamination from the surface by the soils and rock layers covering the aquifer, and until now, the scientific community has believed that this fossil groundwater was safe from modern contamination, but a recent study has challenged this belief. University of Calgary hydro-geologist Dr. Scott Jasechko led a team that discovered ancient groundwater is not immune to modern-day pollution. This study was recently published in Nature Geoscience.
Dr. Jasechko and his co-researchers measured the amount of radioactive carbon in the water to determine the age of the groundwater from more than 6,000 groundwater wells around the globe. Based on their findings they estimated that the majority of the earth's groundwater is likely fossil groundwater, derived from rain and snow that fell more than 12,000 years ago. Dr. Jasechko and his team estimated that this fossil groundwater accounts for between 42%- 85% of available fresh water within a kilometer of the earth's crust. However, they discovered that half of the groundwater wells in the study also contained detectable levels of tritium, a radioactive isotope that was spread around the globe as a result of thermonuclear bomb testing.
Rain and snow that fell on earth after the 1950s contains tritium. Disturbingly, traces of tritium were found in deep well waters, which indicate that contaminated rain and snow melt of today may be able to mix in with deep fossil groundwater and, in turn, potentially contaminate that ancient water until now scientists believed to be pure. This suggests that contemporary contaminants- chemicals of our modern world may be able to reach deep wells that tap fossil aquifers. "The unfortunate finding is that even though deep wells pump mostly fossil groundwater, many still contain some recent rain and snow melt, which is vulnerable to modern contamination," says Jasechko. "Our results imply that water quality in deep wells can be impacted by the land management decisions we make today."
Groundwater is the only available clean drinking water in many parts of the world, and now we find out it may not be as clean as we assumed. Once contaminated, groundwater is very difficult to clean and often after removal of contaminated plumes only long term abandonment of use to allow for natural attenuation is the only possible course of action. Precious groundwater resources increasingly need to be protected and well managed to allow for sustainable long-term use.